Manufacturing process for improved discharge of lithium-containing electrochemical cells

ABSTRACT

In primary cells, the addition of gaseous carbon dioxide to the nonaqueous electrolyte has beneficial effects in terms of minimizing or eliminating voltage delay and reducing Rdc build-up when the cell is subjected to pulse current discharge conditions. For secondary systems, carbon dioxide provided in the electrolyte benefits cycling efficiency. The problem is that carbon dioxide readily degases from an electrolyte prepared under an ambient atmosphere. To prevent this, the carbon dioxide-containing electrolyte is prepared and stored in a carbon dioxide atmosphere. Also, the thusly prepared electrolyte is filed into the casing in a carbon dioxide-containing atmosphere. This prevents degassing of the additive from the electrolyte.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] This invention generally relates to the conversion of chemicalenergy to electrical energy. More particularly, the present inventionpertains to primary lithium/silver vanadium oxide (Li/SVO) cells withimproved voltage delay characteristics and lithium-containing secondarycells with improved cycling efficiency. According to the presentinvention, the activating electrolyte for these electrochemical systemsis provided with a gaseous additive for the purpose of beneficiallymodifying the lithium passivation film. Additionally, electrolytepreparation is carried out in an atmosphere of the gaseous material.This ensures that the intended concentration of the additive in theelectrolyte is maintained. Furthermore, lithium-containing primary andsecondary cells are activated by filling the additive-containingelectrolyte into the casing in the same gaseous additive atmosphere.These procedures significantly improve the voltage delay characteristicsof a Li/SVO cell and the cycling efficiency characteristics of alithium-containing secondary cell in comparison to prior art cells ofsimilar chemistries because very little, if any, of the gaseous additiveevaporates from the electrolyte. Essentially, the intended concentrationof addition is maintained in the electrolyte to improve the voltagedelay and cycling characteristics of the respective cells.

[0003] 2. Prior Art

[0004] Many implantable medical devices use a power source whichconsists, in part, of a primary lithium electrochemical cell. In mostinstances, the primary cell is of a lithium/silver vanadium oxidecouple. This chemistry has proven to be a reliable and dependable powersource which can, if needed, deliver a pulse current discharge such asis required by implantable cardiac defibrillators.

[0005] Recently, the pulse current discharge of Li/SVO electrochemicalcells has been improved by the provision of various types of additivesto the electrochemical chemistry. These additives help alleviate, and insome cases eliminate, the voltage delay phenomenon present duringvarious stages of cell discharge. Voltage delay manifests itself as atemporary decrease in cell voltage during application of a pulsecurrent. It is generally attributed to an increase in the resistance ofthe lithium passivation layer on the surface of the anode electrodewhich impedes the flow of lithium ions from the anode into theelectrolyte during pulse current discharge and results in a temporarilylower voltage exhibited by the cell. In some instances, the life of theimplantable device may be severely reduced.

[0006] Modification of the anode passivation layer by the inclusion ofan additive in the electrochemical chemistry results in the formation ofan ionically conductive protective film thereon. This protective filmgreatly reduces, or even eliminates, the voltage delay phenomenon, andis primarily accomplished by the formation of a salt of one of a numberof additives classified as nitrites, nitrates, carbonates, dicarbonates,phosphonates, phosphates, sulfates, and sulfites on the surface of thelithium metal. The resulting salt is more conductive than lithium oxidewhich may form on the anode in the absence of the electrolyte additive.In fact, it is believed that the lithium additive salt or lithium saltof the additive reduction product on the surface of the anode providesfor the existence of charge delocalization due to resonanceequilibration at the anode surface. This equilibration allows lithiumions to travel easily from one molecule to the other via a lithium ionexchange mechanism. As a result, beneficial ionic conductance isrealized.

[0007] Several methods of beneficially modifying the lithium passivationfilm have been shown to be successful. These include exposing freshlyscraped lithium metal to a gaseous form of the additive prior toinclusion of the active material into the cell assembly, providing asolid form of the additive, when appropriate, in the electrolyte ordissolving a gaseous form of the additive into the electrolyte. Theseand other methods may be employed alone or in combination with eachother.

[0008] One method in particular is preferred because of its lower cost.This involves saturating the electrolyte with a gaseous form of theadditive. For example, when the additive is a carbonate, carbon dioxideis easily saturated into the electrolyte. The modified passivation layeron the surface of the anode electrode forms in-situ when and immediatelyafter the cell is filled with the electrolyte.

[0009] Although process economics favor the use of an electrolyte thatis saturated with gaseous carbon dioxide for alleviating the voltagedelay phenomenon, application of this method to defibrillator batterieshas yet to occur. Previously, lithium cells activated with such anelectrolyte exhibited inconsistent improvements in alleviating thevoltage delay phenomenon. Although lithium cells containing a nonaqueouselectrolyte saturated with carbon dioxide do not exhibit worsenedvoltage delay than historically observed, improvements are notconsistently observed from one cell to the next and from batch to batch.Such unpredictability is not acceptable for cells intended to powerimplantable medical devices, such as cardiac defibrillators.

[0010] Concentration measurements suggest that the inconsistent effectsare due to the processes used to make and store the electrolyte and tofill the cell. Specifically, it has been determined that gaseous carbondioxide degasses from the electrolyte during storage and during vacuumfilling of the cell. The degassing effect becomes more pronounced overtime and results in a significant difference in concentration levelsfrom lot to lot, and even in cells activated with the same electrolytelot. In essence, cells activated with the same electrolyte lot containvarying amounts of dissolved carbon dioxide, and this results ininconsistent alleviation of the voltage delay phenomenon. This alsoresults in variations in the cycling efficiency from one secondary cellto the next.

[0011] The reason for this in a primary cell is that during theapplication of a series of pulse currents, the passivation layer on theanode electrode is disrupted, resulting in the loss of some lithiumcarbonate species from the surface film. After the pulsing is completed,the passivation layer reforms using an additional amount of dissolvedcarbon dioxide within the electrolyte to re-form the lithium carbonatesurface species via reaction with freshly exposed lithium metal. Whenall of the dissolved carbon dioxide is reacted, which occurs prematurelyin cells filled towards the end of an electrolyte lot and from whichappreciable amounts of dissolved carbon dioxide have degassed, thelithium carbonate species no longer forms within the passivation layer.The traditional passivation layer is then present and the alleviation orelimination of the voltage delay phenomenon is no longer possible. Asimilar phenomena occurs in the cycling of a lithium-containingsecondary cell.

[0012] Accordingly, there is needed a method for introducing a gaseousadditive into the electrolyte intended for activating a lithiumelectrochemical cell, of either the primary or the secondary types, sothat the intended concentration of additive is maintained throughoutelectrolyte preparation and filling of the casing. Maintaining theintended additive concentration in the electrolyte ensures alleviation,and in some cases elimination, of voltage delay throughout the usefullife of the cell.

[0013] These and other objects of the present invention will becomeincreasingly more apparent to those skilled in the art by reference tothe following description.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0014] As used herein, the term “pulse” means a short burst ofelectrical current of a significantly greater amplitude than that of aprepulse current immediately prior to the pulse. A pulse train consistsof at least two pulses of electrical current delivered in relativelyshort succession with or without open circuit rest between the pulses. Atypical pulse current ranges from about 15.0 mA/cm² to about 30.0mA/cm².

[0015] The electrochemical cell of the present invention is of either aprimary chemistry or a secondary, rechargeable chemistry. For both theprimary and secondary types, the cell comprises an anode active metalselected from Groups IA, IIA and IIIB of the Periodic Table of theElements, including lithium, sodium, potassium, etc., and their alloysand intermetallic compounds including, for example, Li-Si, Li-Al, Li-Band Li-Si-B alloys and intermetallic compounds. The preferred metalcomprises lithium. An alternate negative electrode comprises a lithiumalloy, such as lithium-aluminum alloy. The greater the amount ofaluminum present by weight in the alloy, however, the lower the energydensity of the cell.

[0016] For a primary cell, the anode is a thin metal sheet or foil ofthe lithium material, pressed or rolled on a metallic anode currentcollector, i.e., preferably comprising nickel, to form the negativeelectrode. In the exemplary cell of the present invention, the negativeelectrode has an extended tab or lead of the same material as thecurrent collector, i.e., preferably nickel, integrally formed therewithsuch as by welding and contacted by a weld to a cell case of conductivematerial in a case-negative electrical configuration. Alternatively, thenegative electrode may be formed in some other geometry, such as abobbin shape, cylinder or pellet to allow an alternate low surface celldesign.

[0017] In secondary electrochemical systems, the anode or negativeelectrode comprises an anode material capable of intercalating andde-intercalating the anode active material, such as the preferred alkalimetal lithium. A carbonaceous negative electrode comprising any of thevarious forms of carbon (e.g., coke, graphite, acetylene black, carbonblack, glassy carbon, etc.) which are capable of reversibly retainingthe lithium species, is preferred for the anode material. A “hairycarbon” material is particularly preferred due to its relatively highlithium-retention capacity. “Hairy carbon” is a material described inU.S. Pat. No. 5,443,928 to Takeuchi et al., which is assigned to theassignee of the present invention and incorporated herein by reference.Graphite is another preferred material. Regardless of the form of thecarbon, fibers of the carbonaceous material are particularlyadvantageous because they have excellent mechanical properties whichpermit them to be fabricated into rigid electrodes that are capable ofwithstanding degradation during repeated charge/discharge cycling.Moreover, the high surface area of carbon fibers allows for rapidcharge/discharge rates.

[0018] A typical negative electrode for a secondary cell is fabricatedby mixing about 90 to 97 weight percent “hairy carbon” or graphite withabout 3 to 10 weight percent of a binder material, which is preferably afluoro-resin powder such as polytetrafluoroethylene (PTFE),polyvinylidene fluoride (PVDF), polyethylenetetrafluoroethylene (ETFE),polyamides, polyimides, and mixtures thereof. This negative electrodeadmixture is provided on a current collector such as of a nickel,stainless steel, or copper foil or screen by casting, pressing, rollingor otherwise contacting the admixture thereto.

[0019] In either the primary cell or the secondary cell, the reaction atthe positive electrode involves conversion of ions which migrate fromthe negative electrode to the positive electrode into atomic ormolecular forms. For a primary cell, the cathode active materialcomprises at least a first transition metal chalcogenide constituentwhich may be a metal, a metal oxide, or a mixed metal oxide comprisingat least a first and a second metals or their oxides and possibly athird metal or metal oxide, or a mixture of a first and a second metalsor their metal oxides incorporated in the matrix of a host metal oxide.The cathode active material may also comprise a metal sulfide.

[0020] The metal oxide or the mixed metal oxide can be produced by thechemical addition, reaction, or otherwise intimate contact of variousmetal oxides and/or metal elements, preferably during thermal treatmentor chemical vapor deposition in mixed states. The active materialsthereby produced contain metals, oxides and sulfides of Groups IB, IIB,IIIB, IVB, VB, VIB, VIIB, and VIII of the Periodic Table of Elements,which includes the noble metals and/or other oxide compounds.

[0021] By way of illustration, and in no way intended to be limiting, anexemplary cathode active material comprises silver vanadium oxide havingthe general formula Ag_(x)V₂O_(y) in any one of its many phases, i.e.β-phase silver vanadium oxide having in the general formula x=0.35 andy=5.18, γ-phase silver vanadium oxide having in the general formulax=0.80 and y=5.4 and ε-phase silver vanadium oxide having in the generalformula x=1.0 and y=5.5, and combination and mixtures of phases thereof.For a more detailed description of silver vanadium oxide materials,reference is made to U.S. Pat. Nos. 4,310,609 to Liang et al., 5,389,472to Takeuchi et al., 5,498,494 to Takeuchi et al. and 5,695,892 toLeising et al., all of which are assigned to the assignee of the presentinvention and incorporated herein by reference.

[0022] Another preferred transition metal oxide useful with the presentinvention is a composite cathode active material that includes V₂O_(z)wherein z≦5 combined with Ag₂O with the silver in either the silver(II),silver(I) or silver(0) oxidation state and CuO with the copper in eitherthe copper(II), copper(I) or copper(0) oxidation state to provide themixed metal oxide having the general formula Cu_(x)Ag_(y)V₂O_(z),(CSVO). Thus, this composite cathode active material may be described asa metal oxide-metal oxide-metal oxide, a metal-metal oxide-metal oxide,or a metal-metal-metal oxide and the range of material compositionsfound for Cu_(x)Ag_(y)V₂O_(z) is preferably about 0.01≦x≦1.0, about0.01≦y≦1.0 and about 5.01≦z≦6.5. Typical forms of CSVO areCu_(1.16)Ag_(0.67)V₂O_(z) with z being about 5.5 andCu_(0.5)Ag_(0.5)V₂O_(z) with z being about 5.75. The oxygen content isdesignated by z since the exact stoichiometric proportion of oxygen inCSVO can vary depending on whether the cathode active material isprepared in an oxidizing atmosphere such as air or oxygen, or in aninert atmosphere such as argon, nitrogen and helium. For a more detaileddescription of this cathode active material, reference is made to U.S.Pat. Nos. 5,472,810 to Takeuchi et al. and 5,516,340 to Takeuchi et al.,both of which are assigned to the assignee of the present invention andincorporated herein by reference.

[0023] Additional cathode active materials for a primary cell includemanganese dioxide, cobalt oxide, nickel oxide, copper vanadium oxide,titanium disulfide, copper oxide, copper sulfide, iron sulfide, irondisulfide, and mixtures thereof.

[0024] In secondary cells, the positive electrode preferably comprises alithiated material that is stable in air and readily handled. Examplesof such air-stable lithiated cathode active materials include oxides,sulfides, selenides, and tellurides of such metals as vanadium,titanium, chromium, copper, molybdenum, niobium, iron, nickel, cobaltand manganese. The more preferred oxides include LiNiO₂, LiMn₂O₄,LiCoO₂, LiCo_(0.92)Sn_(0.08)O₂ and LiCo_(1−x)Ni_(x)O₂.

[0025] To discharge such secondary cells, the lithium metal comprisingthe positive electrode is intercalated into the carbonaceous negativeelectrode by applying an externally generated electrical potential torecharge the cell. The applied recharging electrical potential serves todraw lithium ions from the cathode active material, through theelectrolyte and into the carbonaceous material of the negative electrodeto saturate the carbon. The resulting Li_(x)C₆ negative electrode canhave an x ranging between 0.1 and 1.0. The cell is then provided with anelectrical potential and is discharged in a normal manner.

[0026] An alternate secondary cell construction comprises intercalatingthe carbonaceous material with the active lithium material before thenegative electrode is incorporated into the cell. In this case, thepositive electrode body can be solid and comprise, but not be limitedto, such active materials as manganese dioxide, silver vanadium oxide,titanium disulfide, copper oxide, copper sulfide, iron sulfide, irondisulfide and fluorinated carbon. However, this approach is comprised byproblems associated with handling lithiated carbon outside the cell.Lithiated carbon tends to react when contacted by air or water.

[0027] The above described cathode active materials, whether of aprimary or a secondary chemistry, are formed into an electrode body forincorporation into an electrochemical cell by mixing one or more of themwith a conductive additive such as acetylene black, carbon black and/orgraphite. Metallic materials such as nickel, aluminum, titanium andstainless steel in powder form are also useful as conductive diluentswhen mixed with the above listed active materials. The positiveelectrode of both a primary and a secondary cell further comprises abinder material which is preferably a fluoro-resin powder such aspowdered polytetrafluoroethylene (PTFE) or powdered polyvinylidenefluoride (PVDF). More specifically, a preferred cathode active materialfor a primary cell comprises SVO in any one of its many phases, ormixtures thereof, and/or CSVO mixed with a binder material and aconductive diluent. A preferred cathode active material for a secondarycell comprises lithium cobalt oxide mixed with a binder material and aconductive diluent.

[0028] In that respect, a preferred positive electrode active admixtureaccording to the present invention comprises from about 80% to 99%, byweight, of a cathode active material comprising either one or both ofthe SVO and CSVO materials for a primary cell or lithium cobalt oxidefor a secondary cell mixed with a suitable binder and a conductivediluent. The resulting blended active mixture may be formed into afree-standing sheet prior to being contacted with a current collector toform the subject electrode. The manner in which the electrode mixture isprepared into a free-standing sheet is thoroughly described in U.S. Pat.No. 5,435,874 to Takeuchi et al., which is assigned to the assignee ofthe present invention and incorporated herein by reference. Further,electrode components for incorporation into both primary and secondarycells may also be prepared by rolling, spreading or pressing theelectrode mixture of the present invention onto a suitable currentcollector. Electrodes prepared as described above may be in the form ofone or more plates operatively associated with at least one or moreplates of a counter electrode, or in the form of a strip wound with acorresponding strip of the counter electrode in a structure similar to a“jellyroll”.

[0029] In order to prevent internal short circuit conditions, thepositive electrode is separated from the negative electrode by asuitable separator material. The separator is of electrically insulativematerial, and the separator material also is chemically unreactive withthe negative and positive electrode materials and both chemicallyunreactive with and insoluble in the electrolyte. In addition, theseparator material has a degree of porosity sufficient to allow flowtherethrough of the electrolyte during the electrochemical reaction ofthe cell. Illustrative separator materials include fabrics woven fromfluoropolymeric fibers including polyvinylidine fluoride,polyethylenetetrafluoroethylene, and polyethylenechlorotrifluoroethyleneused either alone or laminated with a fluoropolymeric microporous film,non-woven glass, polypropylene, polyethylene, glass fiber materials,ceramics, a polytetrafluoroethylene membrane commercially availableunder the designation ZITEX (Chemplast Inc.), a polypropylene membranecommercially available under the designation CELGARD (Celanese PlasticCompany, Inc.) and a membrane commercially available under thedesignation DEXIGLAS (C. H. Dexter, Div., Dexter Corp.). The separatormay also be composed of non-woven glass, glass fiber materials andceramic materials.

[0030] The form of the separator typically is a sheet which is placedbetween the negative and positive electrodes and in a manner preventingphysical contact therebetween. Such is the case when the negativeelectrode is folded in a serpentine-like structure with a plurality ofpositive electrode plates disposed between the folds and received in acell casing or when the electrode combination is rolled or otherwiseformed into a cylindrical “jellyroll” configuration.

[0031] The primary and secondary electrochemical cells of the presentinvention further include a nonaqueous, ionically conductiveelectrolyte. The electrolyte serves as a medium for migration of ionsbetween the negative and the positive electrodes during theelectrochemical reactions of the cell, and nonaqueous solvents suitablefor the present invention are chosen so as to exhibit those physicalproperties necessary for ionic transport (low viscosity, low surfacetension and wettability). Suitable nonaqueous solvents are comprised ofan inorganic salt dissolved in a nonaqueous solvent system. For both aprimary and a secondary cell, the electrolyte preferably comprises analkali metal salt dissolved in a mixture of aprotic organic solventscomprising a low viscosity solvent including organic esters, ethers,dialkyl carbonates, and mixtures thereof, and a high permittivitysolvent including cyclic carbonates, cyclic esters, cyclic amides, andmixtures thereof. Low viscosity solvents include tetrahydrofuran (THF),diisopropylether, methyl acetate (MA), diglyme, triglyme, tetraglyme,1,2-dimethoxyethane (DME), 1,2-diethoxyethane (DEE),1-ethoxy,2-methoxyethane (EME), dimethyl carbonate (DMC), diethylcarbonate (DEC), dipropyl carbonate (DPC), ethylmethyl carbonate (EMC),methylpropyl carbonate (MPC), ethylpropyl carbonate (EPC), and mixturesthereof. High permittivity solvents include propylene carbonate (PC),ethylene carbonate (EC), butylene carbonate (BC), acetonitrile, dimethylsulfoxide, dimethyl formamide, dimethyl acetamide, γ-valerolactone,γ-butyrolactone (GBL), N-methyl-pyrrolidinone (NMP), and mixturesthereof.

[0032] The preferred electrolyte for both a primary and a secondary cellcomprises a lithium salts selected from the group of LiPF₆, LiBF₄,LiAsF₆, LiSbF₆, LiClO₄, LiAlCl₄, LiGaCl₄, LiC(SO₂CF₃)₃, LiN(SO₂CF₃)₂,LiSCN, LiO₃SCF₂CF₃, LiC₆F₅SO₃, LiO₂CCF₃, LiSO₃F, LiNO₃, LiB(C₆H₅)₄,LiCF₃SO₃, and mixtures thereof. Suitable salt concentrations typicallyrange between about 0.8 to 1.5 molar.

[0033] In the present invention, the preferred primary electrochemicalcell has a negative electrode of lithium metal and a positive electrodeof the transition mixed metal oxide AgV₂O₅ ₅ (SVO). For this primarycouple, the preferred activating electrolyte is 1.0M to 1.4M LiAsF₆dissolved in an aprotic solvent mixture comprising at least one of theabove listed low viscosity solvents and at least one of the above listedhigh permittivity solvents. The preferred aprotic solvent mixturecomprises a 50/50 mixture, by volume, of propylene carbonate and1,2-dimethoxyethane.

[0034] A preferred electrolyte for a secondary cell of a carbon/LiCoO₂couple comprises a solvent mixture of EC:DMC:EMC:DEC. Most preferredvolume percent ranges for the various carbonate solvents include EC inthe range of about 20% to about 50%; DMC in the range of about 12% toabout 75%; EMC in the range of about 5% to about 45%; and DEC in therange of about 3% to about 45%. In a preferred form of the presentinvention, the electrolyte activating the cell is at equilibrium withrespect to the ratio of DMC:EMC:DEC. This is important to maintainconsistent and reliable cycling characteristics. It is known that due tothe presence of low-potential (anode) materials in a charged cell, anun-equilibrated mixture of DMC:DEC in the presence of lithiated graphite(LiC₆˜0.01 V vs Li/Li⁺) results in a substantial amount of EMC beingformed. When the concentrations of DMC, DEC and EMC change, the cyclingcharacteristics and temperature rating of the cell change. Suchunpredictability is unacceptable. This phenomenon is described in detailin U.S. patent application Ser. No. 09/669,936, filed Sep. 26, 2000,which is assigned to the assignee of the present invention andincorporated herein by reference. Electrolytes containing the quaternarycarbonate mixture of the present invention exhibit freezing points below−50° C., and lithium ion secondary cells activated with such mixtureshave very good cycling behavior at room temperature as well as very gooddischarge and charge/discharge cycling behavior at temperatures below−40° C.

[0035] According to the present invention, electrolyte additivedegassing is prevented by saturating the electrolyte with carbon dioxidewhile the preparation vessel resides in a gaseous carbon dioxideatmosphere. According to another embodiment of the present invention,the carbon dioxide saturated electrolyte is stored in a gaseous carbondioxide atmosphere having a partial pressure sufficient to preventevaporation of the gaseous additive. According to still anotherembodiment, electrochemical cells, of either a primary or a secondarytype, are vacuum filled with the additive-containing electrolyte withinthe confines of a gaseous carbon dioxide atmosphere. The use of air orother inert gases such as helium, argon or nitrogen for preparation orstorage of the electrolyte or for filling cells is unacceptable ascarbon dioxide will still degas from the electrolyte to some extent.

[0036] The use of a carbon dioxide atmosphere greatly reduces or eveneliminates lot to lot variations of dissolved carbon dioxide duringpreparation and storage of the electrolyte. Subsequently, the carbondioxide concentration of the electrolyte within cells is more uniformand consistent than that known by the prior art. Improved cellperformance with respect to alleviation or elimination of the voltagedelay phenomenon results in an improved lithium/silver vanadium oxideprimary cell, which can be used to power implantable cardiacdefibrillators and the like. A carbon dioxide saturated electrolyte alsoresults in a secondary cell with good cycling efficiency in comparisonto secondary cells of the prior art.

[0037] The assembly of the primary and secondary cells described hereinis preferably in the form of a wound element configuration. That is, thefabricated negative electrode, positive electrode and separator arewound together in a “jellyroll” type configuration or “wound elementcell stack” such that the negative electrode is on the outside of theroll to make electrical contact with the cell case in a case-negativeconfiguration. Using suitable top and bottom insulators, the wound cellstack is inserted into a metallic case of a suitable size dimension. Themetallic case may comprise materials such as stainless steel, mildsteel, nickel-plated mild steel, titanium, tantalum or aluminum, but notlimited thereto, so long as the metallic material is compatible for usewith components of the cell.

[0038] The cell header comprises a metallic disc-shaped body with afirst hole to accommodate a glass-to-metal seal/terminal pin feedthroughand a second hole for electrolyte filling. The glass used is of acorrosion resistant type having up to about 50% by weight silicon suchas CABAL 12, TA 23, FUSITE 425 or FUSITE 435. The positive terminal pinfeedthrough preferably comprises titanium although molybdenum, aluminum,nickel alloy, or stainless steel can also be used. The cell header istypically of a material similar to that of the case. The positiveterminal pin supported in the glass-to-metal seal is, in turn, supportedby the header, which is welded to the case containing the electrodestack. The cell is thereafter filled with the electrolyte solutiondescribed hereinabove and hermetically sealed such as by close-welding astainless steel ball over the fill hole, but not limited thereto.

[0039] The above assembly describes a case-negative cell, which is thepreferred construction for either the exemplary primary or secondarycell of the present invention. As is well known to those skilled in theart, the exemplary primary and secondary electrochemical systems of thepresent invention can also be constructed in case-positiveconfigurations.

[0040] It is appreciated that various modifications to the presentinventive concepts described herein may be apparent to those of ordinaryskill in the art without departing from the spirit and scope of thepresent invention as defined by the herein appended claims.

What is claimed is:
 1. An electrochemical cell comprising a negativeelectrode, a positive electrode and a nonaqueous electrolyte, theimprovement comprising: the negative electrode comprised of lithium orthe positive electrode comprised of a lithiated material and thenonaqueous electrolyte being saturated with carbon dioxide at thebeginning of cell discharge.
 2. The electrochemical cell of claim 1wherein the carbon dioxide saturated electrolyte is characterized ashaving been prepared in a container residing in a gaseous carbon dioxideatmosphere.
 3. The electrochemical cell of claim 1 wherein the carbondioxide saturated electrolyte is characterized as having been stored ina gaseous carbon dioxide atmosphere after its preparation but beforebeing used to activate the electrochemical cell, wherein the gaseouscarbon dioxide atmosphere has a partial pressure sufficient to preventevaporation of the carbon dioxide from the electrolyte.
 4. Theelectrochemical cell of claim 1 wherein the carbon dioxide saturatedelectrolyte is characterized as having been filled into a casing housingthe negative electrode and the positive electrode within the confines ofa gaseous carbon dioxide atmosphere.
 5. The electrochemical cell ofclaim 1 as either a primary or a secondary electrochemical cell.
 6. Theelectrochemical cell of claim 1 wherein the electrochemical cell is aprimary cell and the negative electrode is comprised of lithium or alithium-aluminum alloy.
 7. The electrochemical cell of claim 1 as aprimary cell and the cathode active material is selected from the groupconsisting of silver vanadium oxide, copper silver vanadium oxide,manganese dioxide, cobalt oxide, nickel oxide, copper oxide, coppersulfide, iron sulfide, iron disulfide, titanium disulfide, coppervanadium oxide, and mixtures thereof.
 8. The electrochemical cell ofclaim 1 wherein the electrolyte comprises a first solvent selected froman ester, an ether, a dialkyl carbonate, and mixtures thereof, and asecond solvent selected from a cyclic carbonate, a cyclic ester, acyclic amide, and mixtures thereof.
 9. The electrochemical cell of claim1 wherein the nonaqueous electrolyte comprises a first solvent selectedfrom the group consisting diisopropylether, 1,2-dimethoxyethane,1,2-diethoxyethane, 1-ethoxy,2-methoxyethane, dimethyl carbonate,diethyl carbonate, dipropyl carbonate, ethylmethyl carbonate,methylpropyl carbonate, ethylpropyl carbonate, methyl acetate,tetrahydrofuran, diglyme, triglyme, a tetraglyme, and mixtures thereof.10. The electrochemical cell of claim 1 wherein the nonaqueouselectrolyte comprises a second solvent selected from the groupconsisting of propylene carbonate, ethylene carbonate, butylenecarbonate, γ-valerolactone, γ-butyrolactone, N-methylpyrrolidinone,dimethyl sulfoxide, acetonitrile, dimethyl formamide, dimethylacetamide, and mixtures thereof.
 11. The electrochemical cell of claim 1wherein the electrolyte is selected from the group consisting of LIPF₆,LiAsF₆, LiSbF₆, LiBF₄, LiClO₄, LiAlCl₄, LiGaCl₄, LiC(SO₂CF₃)₃,LiN(SO₂CF₃)₂, LiSCN, LiO₃SCF₂CF₃, LiC₆F₅SO₃, LiO₂CCF₃, LiSO₃F, LiNO₃,LiB(C₆H₅)₄, LiCF₃SO₃, and mixtures thereof.
 12. The electrochemical cellof claim 1 wherein the positive electrode comprises from about 80 toabout 99 weight percent of the cathode active material.
 13. Theelectrochemical cell of claim 12 wherein the positive electrode furthercomprises a binder material and a conductive additive.
 14. Theelectrochemical cell of claim 13 wherein the binder material is afluro-resin powder.
 15. The electrochemical cell of claim 13 wherein theconductive additive is selected from the group consisting of carbon,graphite powder, acetylene black, titanium powder, aluminum powder,nickel powder, stainless steel powder, and mixtures thereof.
 16. Theelectrochemical cell of claim 1 as a secondary cell and the cathodeactive material is selected from the group consisting of oxides,sulfides, selenides, and tellurides of metals selected from the groupconsisting of vanadium, titanium, chromium, copper, molybdenum, niobium,iron, nickel, cobalt, manganese, and mixtures thereof.
 17. Theelectrochemical cell of claim 1 as a secondary cell and the anodematerial is selected from the group consisting of coke, graphite,acetylene black, carbon black, glassy carbon, hairy carbon, and mixturesthereof.
 18. The electrochemical cell of claim 1 wherein the electrolytecomprises at least one linear carbonate selected from the groupconsisting of dimethyl carbonate, diethyl carbonate, dipropyl carbonate,ethylmethyl carbonate, methylpropyl carbonate, ethylpropyl carbonate,and mixtures thereof.
 19. The electrochemical cell of claim 18 whereinthe electrolyte comprises at least three of the linear carbonates. 20.The electrochemical cell of claim 1 wherein the electrolyte comprises atleast one cyclic carbonate selected from the group consisting ofethylene carbonate, propylene carbonate, butylene carbonate, vinylenecarbonate, and mixtures thereof.
 21. The electrochemical cell of claim 1wherein the electrolyte comprises ethylene carbonate and an equilibratedmixture of dimethyl carbonate, ethylmethyl carbonate and diethylcarbonate.
 22. The electrochemical cell of claim 21 wherein the ethylenecarbonate is in the range of about 20% to about 50%, the dimethylcarbonate is in the range of about 12% to about 75%, the ethylmethylcarbonate is in the range of about 5% to about 45%, and the diethylcarbonate is in the range of about 3% to about 45%, by volume.
 23. Theelectrochemical cell of claim 1 wherein the activated negative electrodeand positive electrode provide the electrochemical cell dischargeable todeliver at least one pulse current of an electrical current of a greateramplitude than that of a prepulse current immediately prior to thepulse.
 24. The electrochemical cell of claim 15 wherein the pulsecurrent ranges from about 15.0 mA/cm² to about 30.0 mA/cm².
 25. Theelectrochemical cell of claim 1 associated with an implantable medicaldevice powered by the cell.
 26. In combination with an implantablemedical device requiring at least one pulse current for a medical deviceoperating function, an electrochemical cell which is dischargeable todeliver the pulse current, the cell which comprises: a) a negativeelectrode; b) a positive electrode; and c) a nonaqueous electrolyteactivating the negative electrode and positive electrode, wherein thenegative electrode is comprised of lithium or the positive electrode iscomprised of a lithiated material and the nonaqueous electrolyte beingsaturated with carbon dioxide at the beginning of cell discharge.
 27. Amethod for providing an electrochemical cell, comprising the steps of:a) providing a casing; b) housing a positive electrode and a negativeelectrode inside the casing, wherein at least one of the positiveelectrode and the negative electrode is comprised of lithium; c)preparing an electrolyte comprising at least one organic solvent havingcarbon dioxide provided therein in a desired concentration, wherein theelectrolyte is prepared in a carbon dioxide atmosphere having a partialpressure greater that a partial pressure of the carbon dioxide providedin the organic solvent in the desired concentration; d) activating thepositive electrode and the negative electrode with the electrolyte; ande) sealing the casing.
 28. The method of claim 27 including filling theelectrolyte in the casing in the carbon dioxide atmosphere.
 29. Themethod of claim 27 including providing the cell as either a primary or asecondary electrochemical cell.
 30. The method of claim 27 includingproviding the electrochemical cell as a primary cell and the negativeelectrode is comprised of lithium or a lithium-aluminum alloy.
 31. Themethod of claim 27 including providing the electrochemical cell as aprimary cell and selecting the cathode active material from the groupconsisting of silver vanadium oxide, copper silver vanadium oxide,manganese dioxide, cobalt oxide, nickel oxide, copper oxide, coppersulfide, iron sulfide, iron disulfide, titanium disulfide, coppervanadium oxide, and mixtures thereof.
 32. The method of claim 27 whereinthe activated negative electrode and positive electrode provide theelectrochemical cell dischargeable to deliver at least one pulse currentof an electrical current of a greater amplitude than that of a prepulsecurrent immediately prior to the pulse.
 33. The method of claim 27including providing the electrochemical cell as a secondary cell andselecting the cathode active material from the group consisting ofoxides, sulfides, selenides, and tellurides of metals selected from thegroup consisting of vanadium, titanium, chromium, copper, molybdenum,niobium, iron, nickel, cobalt, manganese, and mixtures thereof.